Exposing the mechanism and timing of impact of nonindigenous species on native species

The nonnative mudsnail Batillaria attramentaria was introduced to the west coast of North America in the early part of this century and has been displ acing the native mudsnail Cerithidea californica on a marsh-by-marsh basis. We combined detailed, quantitative field data on this invader and its inte ractions with the native snail in an individual-based model that allowed us to address both general questions about the mechanism of displacement of n ative species and more specific concerns about detecting the trajectory and impact of the invasion. In empirically parameterized simulations the nativ e snail was driven extinct within 55-70 yr after introduction of Batillaria , which closely matches direct field estimates. We then tested the relative importance of Batillaria's demonstrated advantages in parasitism resistanc e (top-down effect), exploitative competition (bottom-up effect), and morta lity rate (demographic advantage) in driving its displacement of Certithide a. With its demographic advantage maintained, but without its advantages in competition and parasitism, Batillaria still drove Cerithidea extinct with in 90 yr. Only when Batillaria's mortality rate was set equal to that of Ce rithidea could the native snail persist indefinitely, demonstrating this fa ctor's overwhelming influence on the success of this invasion. The differen ce in mortality between the species was large relative to the other differe nces, but further simulations showed that the importance of this difference stems not just from its magnitude, but also from the sensitivity of this s ystem to this demographic rate. Identification of the relative importance o f mechanisms that contribute to an invader's success is one of the major be nefits of such modleing efforts. To identify empirically measurable quantities that provide the earliest war ning of impact on the native species, we tracked many population- and indiv idual-level responses of Cerithidea to Batillaria's invasion, including pop ulation density, biomass, egg production, mean size, proportion of infected individuals, and individual growth rate, as well as availability of shared food resources. We used the empirically observed parameter values and an i nitial number of Batillaria invaders in these: simulations that guaranteed extinction of Cerithidea within 90 yr. Despite a rapid initial increase in invader populations, all metrics for Cerithidea were slow to exhibit signs of impact. Most took at least 25 yr from invasion to exhibit detectable cha nges, by which time the nonnative species was established at high densities (> 3000 snails/m(2)). Certithidea egg production was the fastest, most con sistent response metric exhibiting declines within 20-25 yr after invasion in similar to 90% of simulations. Difficulty in finding reliable, early war ning metrics has crucial implications for how we should view and conduct mo nitoring programs and risk assessment analyses.